It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as a reducing agent being added in a position between the selective reduction catalyst and the particulate filter, thereby attaining lessening of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted in the position between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between the urea-water added position and the selective reduction catalyst. However, arrangement of the particulate filter and the selective reduction catalyst in a substantially spaced apart relationship will extremely impair the mountability thereof on a vehicle.
In order to overcome this, an exhaust emission control device compact in size as shown in FIG. 1 has been proposed by the applicant same as that of the present invention (see below-mentioned Patent Literature 1). In the exhaust emission control device illustrated, incorporated in an exhaust pipe 2 through which exhaust gas 1 from an engine flows is a particulate filter 3 housed in a casing 5 to capture particles in the exhaust gas 1; arranged downstream of and in parallel with the particulate filter 3 and housed in a casing 6 is a selective reduction catalyst 4 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. An exit side of the particulate filter 3 is connected to an entry side of the selective reduction catalyst 4 through an S-shaped communication passage 7 such that the exhaust gas 1 discharged from the exit side of the particulate filter 3 is oppositely turned into the entry side of the adjacent selective reduction catalyst 4.
The communication passage 7 is the S-shaped structure comprising a gas gathering chamber 7A which encircles an exit end of the particulate filter 3 to gather the exhaust gas 1 just discharged therefrom through substantially perpendicular turnabout of the gas, a communication pipe 7B which extracts the gathered exhaust gas 1 from the chamber 7A oppositely to the flow of the exhaust in the particulate filter 3 and a gas dispersing chamber 7C which encircles the entry side of the selective reduction catalyst 4 to disperse the exhaust gas 1 guided by the communication pipe 7B through substantially perpendicular turnabout of the gas into the entry side of the selective reduction catalyst 4. An entry end of the communication pipe 7B is centrally provided with an injector 8 for addition of the urea water into the communication pipe 7B and directed toward the exit side thereof.
In the example illustrated, arranged in the casing 5 and in front of the particulate filter 3 is an oxidation catalyst 9 for oxidization treatment of unburned components in the exhaust gas 1, and arranged in the casing 6 and behind the selective reduction catalyst 4 is an ammonia lessening catalyst 10 for oxidization treatment of surplus ammonia.
With such construction being employed, particulates in the exhaust gas 1 are captured by the particulate filter 3; downstream thereof and at the entry end of the communication pipe 7B, the urea water is added into the exhaust gas 1 by the injector 8 and is pyrolyzed into ammonia and carbon dioxide gas so that NOx in the exhaust gas 1 is favorably reduced and depurated by the ammonia on the selective reduction catalyst 4. Thus, both the particulates and NOx in the exhaust gas 1 are lessened.
In this case, the exhaust gas 1 discharged from the exit side of the particulate filter 3 is oppositely turned by the communication passage 7 into the entry side of the adjacent selective reduction catalyst 4 so that a long distance is ensured between the urea-water added position and the selective reduction catalyst 4 to ensure enough reaction time for production of ammonia from the urea water.
Moreover, the particulate filter 3 is arranged in parallel with the selective reduction catalyst 4, and the communication passage 7 is arranged between and along the particulate filter 3 and selective reduction catalyst 4 so that the whole structure becomes compact in size to substantially enhance mountability thereof on a vehicle.
And, as disclosed in the below-mentioned Patent Literature 1, employed in a position where urea water is added by the injector 8 is a mixer structure 15 such that the exhaust gas 1 from the gas gathering chamber 7A is tangentially introduced into entry-side openings 11 on the cylindrical communication pipe 7B by guide fins 12, 13 and 14 as particularly shown in FIGS. 2 and 3, thereby affording a swirling flow (swirl) to the exhaust gas 1. The urea water is added centrally of the swirling flow by the injector 8.
Specifically, in order to further effectively facilitate transformation of the urea water into ammonia, it is necessary to positively facilitate gasification of the urea water to proceed with substantive chemical reaction for ammonia production. Thus, the swirling flow is afforded to the exhaust gas 1 and the urea water is added from the injector 8 centrally of the swirling flow, whereby the urea water is effectively contacted with an inner periphery of the communication pipe 7B to facilitate gasification of the urea water through heat receiving from the inner periphery of the communication pipe 7B.